Preservation of food materials is a problem which continually confronts the food industry, particularly the pet food industry where canning methods are not in all instances possible or desirable. In the pet food industry, and also in the human food industry, food materials are commonly marketed in three forms: dry products, intermediate moisture products, and high moisture products. Dry products are commonly cereal type materials and due to their low moisture content, about 10% by weight or less, they are very stable and resist mold growth and bacterial spoilage. Intermediate moisture products have a moisture content of about 25 to 40% by weight. Due to the higher moisture content of the intermediate moisture products, they must be stabilized with preservatives to provide sufficient bacteriastatic and antimycotic activity in the material to give it an acceptable shelf life. Sugar, salt, sodium and potassium sorbate, sorbic acid and many other preservative materials have been used to preserve intermediate moisture materials. Due to the high stability of dry and intermediate moisture products, they can be packaged in relatively inexpensive paper or polymer film containers and kept in an ambient environment for long periods of time without spoiling. The materials do not require refrigeration after packaging.
High moisture products such as canned pet food have a moisture content of from about 50 to 80% or more by weight. The high moisture level of the material has required that the material be sterilized after canning in order to prevent spoilage from bacterial and mold growth if a shelf life of more than a few days is desired. The ease with which high moisture material spoils has required that the product be sterilized and remain sealed in the can if it is stored at room conditions. Once a can of high moisture product is opened, it spoils rapidly unless it is refrigerated.